I have a house built in the mid-70s in New York. One section of the house has electric baseboard heat. I have replaced the thermostat with a 24V digital. The relay is the original, so it is getting close to 40 years old and I would like to replace it before it fails, although it is functioning normally at the moment. I took a picture of the old relay and a picture of one that I purchased. Should I use the new relay or should I get a different one? Thanks for the help.

If the wiring is the same, you can replace it, but the old one is rated for 220V/25A while the new one is only rated for 220V/22A They are close, but some headroom is always a good idea, so staying with 25A or larger would be my suggestion based on information given.

If you knew the total current draw by all heaters in the circuit, that would help a great deal.

Ideally, and electrician should do the installation, as you are controlling a great deal of power, fire is likely if something is done wrong. Home Insurance companies will often not pay if total loss was caused by owner modifications that weren't installed to code. The problem is knowing what "to code" is in your area. The North East is by far the most regulated area in the US when it comes to things like that.

Your new part looks like a time delay device. The old, "Klixon" heat relays used a heater to pop a bimetal that closes the contacts. You might not have what you expected.

Hook it up on the 24 volt side and see if it clicks on instantly or has several seconds (some take up to 90 seconds) of delay.
Edit: I looked it up and found a 75 second delay built in.

It might not be easy to get a 25 amp rated controller because of the ever increasing cheapitude of the manufacturers. 5KW will use 21.7 amps at 230V. That will work, but you can still buy 25 or 30 amp relays if you know what you're loking for.

thanks for all the help guys. I have no idea what the load is. All I can add is that there are 3 baseboards on the circuit, 2 X ten feet and 1 X 3 feet if that is any indication of what they would drawn. I think the breaker is a 20 amp.

Honeywell R841C1227 is a 24 V Electric Heater Relay with SPST switching action (single pole, single throw).
The R841C1227 is used with two-wire, 24 Vac thermostat to control electric heating equipment such as baseboard, ceiling cable, duct heaters and can be mounted in any position. It also provides a versatile operation with each cycle of the thermostat(4 to 6 cycles per hour). The small size and silent operation of the R841 allow installation in a living area, utility room or basement.

In furnaces over 5000 watts, they are switched on in stages to minimize light dimming and start surge. In a furnace or heater up to 5000 watts, you don't need to stage the load.

The bother comes when the thermostat manufacturer decides to install a delay which then adds with the delay of the Honeywell "relay", and the brain board delay. In air conditioners, there is a necessary 3 minute re-start delay for the compressor which has to wait until the pressures equalize to start. Suddenly, you find the compressor has a delay, the controller in the air handler has a delay, and the thermostat has a delay, and you stand there for 10 minutes wondering if it's going to start or not because everybody wants to add a start delay, just because they can.

I think that is what the diagram on the old one shows: DPST outputs (2 lines switched) 240V 25A contacts with integrated 24VAC transformer for powering the thermostat circuit if I'm reading it right.

--ETA: Ok, I found a rule of thumb as 250W/ft of baseboard heat. So the OP has a total of 3,250 Watts, or ~15A@220V. Using a DPST, the 10 foot section would need to 12 AFL contacts, and the 3ft would need smaller, 5AFL would be fine.

If all the delays are synchronized, it doesn't improve anything.
However, a bimetal and a heater is not a very accurate timer. Displacing the start events by as little as 2 or 3 seconds will accomplish avoiding side effects.